1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically a semiconductor integrated circuit capable of simply and easily detecting output abnormality of input buffers. In this specification, the term "input buffer" is used to include all buffers which receive an input signal for the semiconductor integrated circuit, and therefore, should be understood to include not only an input buffer used only for receiving an input signal, but also an input/output buffer or a bi-direction buffer having a function of receiving an input signal from an external device and a function of outputting an signal to an external device.
2. Description of Related Art
In the prior art, an input threshold voltage (Vth), which is one parameter of electrical characteristics of semiconductor integrated circuits, has been tested, for example after a step of manufacturing semiconductor integrated circuits. In one typical test method, a function test has been performed by applying to input terminals, a pattern of input voltages which are determined in accordance with types of the input terminals and which correspond to a maximum input low level voltage (V.sub.ILMAX) or a minimum input high level signal (V.sub.IHMIN), both of which must be ensured for the semiconductor integrated circuit. If a normal operation can be found in the function test, it can be concluded that input buffers are normal.
However, the above mentioned conventional method has encountered a problem in which, since noise mixed into a voltage supply line for a measurement system directly adversely influences stability of the measurement system, integrated circuits which are not defective in characteristics have been often deemed to be defective as the result of the function test. Particularly, recent inclination of increase in the number of connection pins and elevation of performance (such as increase of an operation speed and increase of a driving power) has resulted in an increased transient current appearing at outputs of the integrated circuit. This is one cause for the above mentioned noise. In addition, the increase in the number of connection pins has resulted in an increased impedance of a voltage supply line of a test board (IC tester), which has further aggravated generation of the above mentioned noise, and therefore, the above mentioned problem has become more and more conspicuous.